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Athabasca Basin - McClean Lake, Midwest

Saskatchewan, Canada

Main commodities: U Ni Co
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The McClean Lake and Midwest uranium deposits are located at the base of the late Paleo- to Mesoproterozoic Athabasca Basin, approximately 750 km north of Saskatoon in northern Saskatchewan, Canada.   Midwest is approximately 20 km west of McClean Lake.

Both deposits are near the eastern margin of the Athabasca Basin in the Churchill Structural Province of the Canadian Shield. In the area, the flat-lying, unmetamorphosed sandstones and conglomerates of the Mesoproterozoic Athabasca Group (of the Athabasca Basin) unconformably overlie Archaean and Paleoproterozoic gneisses.

The Midwest deposit straddles the transition zone between two prominent litho-structural domains within the Precambrian basement, namely the Wollaston to the east and the Mudjatik to the west. The McClean Lake deposits overlie the Wollaston domain. During the Hudsonian Orogeny intense thermo-tectonic activity remobilised the Archaean crystalline rocks and intensely folded the overlying early Paleoproterozoic supracrustal metasedimentary units. In the Mudjatik domain, which represents the orogenic core, non-linear, felsic, granitoid to gneissic rocks are surrounded by subordinate thin gneissic supracrustal units, which have reached granulite-facies metamorphic grades, usually occurring as broad domal features. The adjacent Wollaston domain, is characterised by a steeply dipping isoclinally folded sequence of Paleoproterozoic gneissic rocks with a distinct NE lineal structural trend.

The basement surface below the Athabasca Basin sediments is a palaeo-weathered regolith with lateritic characteristics. This surface is unconformably overlain by the late Paleo- to Mesoproterozoic clastic sedimentary rocks of the deep, closed and elliptically shaped Athabasca Basin. The sedimentary rocks in the basin are fluvial sandstones and conglomerates with minor shales and dolomites.

The area is cut by a major northeast-striking fault set of the ~1850 Ma Hudsonian Orogeny, predominantly in the basement rocks but also often extending up into the Athabasca Group as the result of periods of post-depositional movement. Dolerite sills and dykes, which are often mineralised, are frequently associated with the faulting.
Midwest

The pre-Athabasca basement underlying the Midwest deposit comprises a steeply-dipping NE trending syncline of locally graphitic and commonly garnetiferous early Paleoproterozoic pelitic metasediments of the Wollaston Group, wedged between a Western Archaean granite and an Eastern granite gneiss believed to represent Archaean basement which was remobilised by the thermo-tectonic Hudsonian Orogeny. The syncline has a northeast trend, parallel to the main shear zone and the regional structure. The upper margin of the metamorphic basement, immediately underlying the unconformity at the base of the Athabasca Basin sediments, is occupied by a partially preserved 0 to 30 m thick 'tropical' palaeo-weathering regolith. The nature and thickness of the regolith depends on the structure and lithologies of the basement on which it was developed. In zones of mineralisation, much the regolith is commonly completely obliterated by hydrothermal alteration.

The Athabasca Sandstone unit covers the whole of the Midwest deposit area. The Athabasca Sandstone thickens westward into the basin and is represented by an up to 200 m thick sequence of sandstones with minor interbedded conglomerates. These sandstones were deposited as alluvial fans and in braided streams and typically show abundant cross-bedding, coarser and finer units, and a general horizontal layering. The lower part of the sandstone sequence, which is approximately 80 m thick, has a much less siliceous matrix and an increased amount of conglomerate bedding.

The sandstones of the Proterozoic Athabasca Basin sequence are covered by surficial deposits, which were laid down during recent glacial episodes and mainly comprise glacial till, which in the Midwest area is locally overlain by glaciofluvial sands and gravels, and recent alluvial sands and silts. The overall thickness of the overburden deposits varies from 4 to 20 m, averaging 15 m.

The pre-Athabasca Basin rocks are structurally complex, having undergone at least three major Hudsonian deformational episodes. Many of the faults show evidence of several superimposed episodes of activity, with both horizontal and vertical movement. Some of the fault sets were reactivated after deposition of the Athabasca Sandstone, providing conduits for hydrothermal activity and loci of mineralisation. In addition, horizontal shear cleavage has been identified at the unconformity surface between the crystalline basement and the Athabasca Sandstone particularly in the zone of the strongest alteration and mineralisation.

Structural control of mineralisation at Midwest is evident at both the regional and local scale. Regionally the deposit is located along a major graphitic trend which corresponds to a zone of extensive faulting, while locally, the ore morphology within the deposits is strongly influenced by high-angle faults. The mineralisation follows a general 30° trending fault corridor, which also parallels to the general strike of the graphitic basement unit. On a secondary scale, within the envelope of mineralisation, high-grade lenses are elongated in both an ENE and locally a NNW direction. In cross-section an average 15 m down-drop to the west is evident across the 30° trending fault set.

The Midwest uranium deposit is hosted at the unconformity and extends for approximately 600 m along a 40° strike direction, ranges from 10 to >128 m, averaging 80 m in width and up to 30 m, averaging 8 m in vertical thickness. The deposit has been divided into three distinct zones: the North, South and the Southern Extension Zones. The North zone in turn is composed of a lower grade segment on its northern extremity and a 140 m long very high grade section in its southern part, which accounts for near 70% of the resource. The South Zone is similarly composed of an 80 m long high grade segment on its northern end, with the remainder being of lower grade.

The high grade North Zone segment in turn comprises two high grade pods that strike at 80°, while the northern high grade section of the South Zone displays trends of both NW and NE. The low grade southern segment of the South Zone and South Extension Zone strike at approximately °. The North and South Zones are separated by a 50 m long section of low-grade disseminated mineralisation. The overall grade of the deposit averages 5.47% U3O8.

The majority of the mineralisation is located at the basement sandstone contact, either in the basal conglomerate of the Athabasca Sandstone, or in the adjacent basement immediately below the unconformity. Approximately 13% of the resource tonnage and 2% of the contained U
3O8 occurs in the overlying sandstones. The mineralisation follows a well defined fault controlled corridor, paralleling the strike of a graphitic basement unit, and has sharp, grade-based, lateral boundaries, in both plan and section view. Locally, mineralised lenses also are found along steep faults, protruding above and below the main unconformity ore envelope. Generally, the distribution of uranium-nickel mineralisation with anomalous cobalt, is controlled by the intersection of graphitic, pelitic metasediments in the basement, the major fault system which parallels and offsets them, and the unconformity, with the main mineralised zone following a series of such faults.

Mineralisation comprises uranium oxides (uraninite and pitchblende) and a suite of nickel-cobalt arsenides, sulphides and sulpharsenides in a clay matrix, with the highest grades occurring as 1 to 4 m thick sections of massive uranium oxides and niccolite surrounded by a thin clay envelope, with average grades of 4.1% Ni, 0.33% Co and 6.8% As. The high grade intervals are typically surrounded by a thin, 1 to 2 m thick, envelope of low-grade 0.05 to 0.5% U
3O8, which consists of massive clay and strongly argillised sandstone.

The mineralisation occurs:

i). At, and immediately above the unconformity, as a series of flat-lying, fault-bounded localised bodies, predominantly composed of massive and colloform pitchblende, with nickel arsenides as well as uraninite and coffinite in a gangue of sideritic and hematitic clay.
ii). Within the Athabasca Sandstone, where mineralisation is present as discrete pods of fine-grained sooty pitchblende and nickel arsenides filling and coating fractures, and as disseminations throughout the sandstone clay matrix. All of the sandstone mineralisation in the resource estimate is located above the North Zone.
iii). Below the unconformity in the basement, as pitchblende and nickel arsenides in veins and disseminations associated with kaolinite or chlorite and sericite, localised in shear zones extending to at least 100 m below the unconformity.

The probable mineral reserve in 2005, as quoted on Denison Mines website in 2007 was:
    0.3455 Mt @ 5.47% U
3O8, 4.37% Ni, 0.33% Co = 18900 t U3O8.

Ore is to be treated at the McClean mine processing plant.
McLean Lake

The McClean Lake mine commenced production in mid 1999 and has operated at a rate of around 2500 tpa U
3O8 from ore grading 2.4% U3O8, exploited from 4 open pits, and a new underground mine. There are a number of separate deposits that comprise the operation, including the Sue A, B, C and E, the McClean North and South, and the JEB deposits. Two of these, JEB and Sue C, have been mined out. JEB is around 9 km NNW of the Sue group. Depths to the unconformity are shallow, rarely exceeding 175 m, favouring open pit mining.

The pre-Athabasca Basin basement geology beneath the McClean Lake area is composed of a thin, 200 to 300 m thick, cover of early Paleoproterozoic gneissic rocks lying on Archaean granitoid gneisses. Approximately half of the McClean Lake area is underlain by the felsic granitoids which occur as domal masses and range from foliated granitoids in the core to more gneissic rocks on the margins. Complex folding has produced thin arcuate antiforms of Archaean granitoids surrounded by narrow synforms of Paleoproterozoic pelitic gneisses containing a dominant graphitic member in its lower sections. All of the known significant uranium mineralisation at McClean Lake are directly associated with that graphitic member.

The upper margin of the metamorphic basement, immediately underlying the unconformity at the base of the Athabasca Basin sediments, is occupied by a partially preserved 0 to 30 m thick 'tropical' palaeo-weathering regolith. The nature and thickness of the regolith depends on the structure and lithologies of the basement on which it was developed. In zones of mineralisation, much the regolith is commonly completely obliterated by hydrothermal alteration. The Athabasca Sandstone unit covers all of the mine area and is represented by up to 200 m of the Manitou Falls formation, a non-marine fluviatile sandstone with conglomeratic lenses in the basal B member. These sandstones were deposited as alluvial fans and in braided streams and typically show abundant cross-bedding, coarser and finer units, and a general horizontal layering.

The Athabasca Sandstone is overlain by Quaternary surficial deposits which largely comprise a Pleistocene drumlin till plain resting directly on the sandstone bedrock. The till is locally overlain in turn by glacio-fluvial sands and gravels, and recent alluvial sands and silts. The till is generally 2 to 4 m thick, but reaches as much as 15 m under gently undulating drumlins that are up to 30 m in relief.

The pre-Athabasca Basin rocks are structurally complex, having undergone at least three major Hudsonian deformational episodes. Many of the faults show evidence of several superimposed episodes of activity, with both horizontal and vertical movement. Some of the fault sets were reactivated after deposition of the Athabasca Sandstone, providing conduits for hydrothermal activity and loci of mineralisation. In addition, horizontal shear cleavage has been identified at the unconformity surface between the crystalline basement and the Athabasca Sandstone particularly in the zone of the strongest alteration and mineralisation.

The mineralised zones in the McClean Trend occur as sausage­shaped pods straddling the unconformity between the Athabasca Sandstone and the crystalline basement. The host rocks are altered Athabasca Sandstone and early Paleoproterozoic basement rocks usually subjected to strong clay alteration. There are 11 discrete pods, arranged along two separate but parallel trends (termed the North and South zones) separated by approximately 500 m. Generally, mineralisation in the basement is on the eastern extremity of the combined zone.

Mineralisation at both the McClean North and South deposits was controlled by a zone of strong east-west to ENE trending, 70°S dipping faults and fractures coinciding with the basement graphitic gneisses. These faults show evidence of a combination of normal and reverse offsets of as much as 20 m. Steeply-dipping NE and NW trending cross fractures with both vertical and lateral displacement have also been mapped. The graphitic basement gneiss, which hosts, or is immediately below, the main ore deposits, is in fault contact to the east with feldspathic gneisses and granitoid rocks, whereas to the west it is gradational with intermediate gneissic units.

These ENE trending faults and gneisses of the McClean Trend may represent a splay from the west extension of the Tent Seal fault that forms the northern contact of the Collins Bay dome with the early Paleoproterozoic intermediate to felsic gneiss, calc-silicates, and quartzites.

The deposits of the Sue Trend, 3 km to the east of the McClean Trend, follow a linear trend on the western flank of the Collins Bay dome. These deposits trend north­south along or near a steeply east­dipping unit of graphitic gneiss within a 4.2 kilometre long basement conductor. Combinations of normal and reverse faults which parallel the east-dipping foliation in the basement graphitic gneisses have produced a basement relief variation of 10 to 20 m, whereby reverse faulting stepped the unconformity down to the west. The Sue A and B deposits occur along the western flank of a basement horst (of the Collins Bay dome) which has 8 to 10 m of relief. The NE and NW striking faults offset and modify the major north-south structural controls, thereby either limiting, or significantly controlling, the extent of mineralisation along the trend. The postion of the mineralisation progresses from predominantly within the Athabasca Sandstones at Sue A to the north, to straddle the unconformity at Sue B, to being entirely within the basement graphitic gneisses at Sue C and E on the southern end of the trend.

At the Sue A deposit, the mineralisation is primarily composed of uranium oxides (uraninite and pitchblende) with a suite of nickel-cobalt arsenides (primarily niccolite) in a hematitic clay matrix. Nickel, cobalt, and arsenic grades are generally <<1%, and high-grade uranium mineralisation is typically surrounded by a thin low-grade envelope (0.05 to 0.5% U
3O8), which is one to two metres thick and consists of massive clay and strongly argillised sandstone.

The Sue B deposit is at a depth of 60 to 100 m below surface and is some 350 m north of Sue A deposit. Like Sue A, it is developed along faults on the western flank of a basement horst which have offset the unconformity by 8 to 10 m. The mineralisation is predominantly fault and fracture controlled and, like Sue A, occurs as a flat lens which straddles the unconformity and is 30 to 90 m long by 2 to 20 m wide and up to 8 m thick.   In additional, normal and reverse movement on faults paralleling the east-dipping gneiss foliation resulted in fracturing of the sandstones in the overlying Athabasca Sandstone to produce a 60 m long, up to 30 m (averaging 10 m) wide and 19 m thick perched lens of mineralisation high in the sandstones at a depth of approximately 17 to 43 m. The unconformity mineralisation lies at depths of around 52 to 82 m. The two lenses are separated by 20 to 35 m of waste, but are connected by vertically extensive mineralisation along feeder faults. The mineralisation at Sue B is primarily composed of the uranium oxides uraninite and pitchblende, accompanied by a suite of nickel-cobalt arsenides (primarily niccolite) enclosed within massive, earthy red clays. The sandstone-hosted upper lens contains remnant silicification and has grades range up to 16.5% U
3O8, while Ni, As and Co grades are variable, with nickel ranging up to 32% and arsenic up to 43%.

The now mined out Sue C deposit which was 100 m west of the south end of Sue A is entirely hosted within the basement below the Athabasca Basin. High grade lenses of the order of 1 to 2 m thick and 3 to 5 m in vertical dimension, situated en echelon within a zone hundreds of metres in strike length and extend down-depth for tens of metres. These pods are bounded by sheared and brecciated graphitic schist that contains other smaller lenses of the same material, forming an envelope of low-grade ore that can be blended with high-grade ore during mining and milling if necessary. The deposit had a strike length of 390 m, trending at 348° and dipped at 75°E. The upper margin of the deposit was limited by the unconformity surface at the base of the Athabasca Sandstone in its northern and central parts and plunged gently to the south in its southern sections. The central part of the deposit, occupies a length of 80 to 100 m, and extended downwards from the unconformity for 80 m, containing approximately 75% of the known reserves. The host rock alteration surrounding the mineralisation is dominantly chloritic with restricted illite at the expense of biotite, cordierite and garnet.

At the Sue E deposit which is to the south of Sue C, mineralisation occurs over a strike length of approximately 320 m, with horizontal widths varying from 4 to 15 m, and is restricted to depths of from 50 m to 200 m below the surface. The deposit is totally basement hosted, occurring as narrow, steeply-dipping vein-type mineralisation, with a relatively clean mineralogy, similar to that of the southern part of the Sue C deposit. Sue E contains more Ni and As than Sue C. The U
3O8 to As ratio is 1.03, and the U3O8 to Ni ratio is 1.57.

The McClean North and South deposits are approximately 3 km WSW of Sue C and are situated between two Archaean basement domes. The deposits are aligned along two trends within a linear belt of graphitic gneisses, as described above. The Athabasca Sandstones are typically 150 to 160 m thick and are covered by 1 to 10 m of glacial overburden. Beneath the sandstones, the regolith varies from 15 to 45 m thick, but it is invariably obliterated in the zones of uranium mineralisation. Uranium mineralisation is hosted in both altered Athabasca Sandstone and in graphitic basement gneiss and is surrounded by hematite and chloritic altered clay­rich zones in which illite forms a mushroom­shaped envelope tilted to the north. The illite clay alteration extends upwards along fractures into the sandstones for tens of metres where it is capped by silicified sandstones). In the basement, alteration consists of bleaching, chloritisation, argillisation and hematisation. Uranium mineralisation in the McClean North trend pods occurs over vertical widths of typically 10 to 20 m. In cross-section the pods are flat, lenticular to oval shaped bodies. The higher grade portions of the pods undulate from 13 above to 12 m below the sandstone­basement contact which is, on average, 160 m below the surface. The uranium occurs as fine­grained coffinite, as veinlets and nodules of pitchblende and as large masses of pitchblende/uraninite. Highly variable but generally small amounts of nickel arsenides are associated with the uranium. Generally, the mineralisation located below the unconformity is cleaner than that found in the sandstone. Deposition and associated alteration appears to have been controlled by strong east-west faulting and fracturing coincident with the basement graphitic gneisses. Highly variable but generally small amounts of nickel arsenides are associated with the uranium. The McClean North deposit has a U
3O8 to arsenic and nickel ratio of 0.20 and 0.11 respectively, while the McClean South ratios are 0.57 and 0.31 respectively. Generally, the mineralisation located below the unconformity has less arsenic and nickel than that found in the sandstone.

The Caribou uranium deposit is approximately 2 km NW of Sue C and is located beneath Caribou Lake. The deposit is approximately 85 m long, 25 m wide, and 10 to 25 m thick. It has been thickened at its centre by reverse fault repetition along an east-west structure, near the intersection with the NE trending faults. Mineralisation consists primarily of the uranium oxides uraninite and pitchblende accompanied by a suite of nickel-cobalt arsenides (niccolite), sulphides and sulpharsenides in a clay altered matrix within the sandstones and fault breccias in the basement. The mineralisation is concentrated along the unconformity between the Athabasca Sandstone and Paleoproterozoic basement and at the contact between the conglomeratic and sandstone units of the Manitou Falls member of the Athabasca Group, particularly where permeable fanglomerate is present. Structural controls of the uranium mineralisation above the unconformity in the Athabasca Basin sediments consist of 30° trending, 65 to 70° east-dipping faults that host medium to high-grade pitchblende veins and low-grade replacement mineralisation. The same faults also control low and medium grade uranium mineralisation in the basement immediately below the unconformity. These faults form a parallel set, each separated by 5 to 10 m with both reverse and normal displacements of <5 m that cut the mineralisation but also host massive to semi massive pitchblende veins. The veins are typically brecciated due to reactivation or protracted movement along the structures, although replacement and open-space filling textures are also evident.

Published reserves and resources in 2005, as quoted on Denison Mines website in 2006 include:

    Sue A, probable reserve - 0.03195 Mt @ 1.99% U
3O8
    Sue A resource at 0.1% U
3O8 cut-off - 0.0817 Mt @ 0.88% U3O8
      Sue A resource at 0.5% U
3O8 cut-off - 0.0372 Mt @1.60% U3O8

    Sue B indicated resource at 0.1% U
3O8 cut-off - 0.0729 Mt @ 0.73% U3O8
      Sue B indicated resource at 0.5% U
3O8 cut-off - 0.0455 Mt @0.97% U3O8
    Sue B inferred resource at 0.1% U
3O8 cut-off - 0.0120 Mt @ 0.95% U3O8
      Sue B inferred resource at 0.5% U
3O8 cut-off - 0.0053 Mt @1.70% U3O8

    Sue C (worked out) production - 0.2499 Mt @ 5.3% U
3O8 for 13265 t U3O8 recovered.

    Sue E probable reserve - 0.6281 Mt @ 0.78% U
3O8
    Sue E indicated resource at 0.1% U
3O8 cut-off - 0.7183 Mt @ 0.775% U3O8
      Sue E indicated resource at 0.5% U
3O8 cut-off - 0.3123 Mt @1.456% U3O8
    Sue E inferred resource at 0.1% U
3O8 cut-off - 0.7803 Mt @ 0.685% U3O8
      Sue E inferred resource at 0.5% U
3O8 cut-off - 0.2097 Mt @1.956% U3O8

    JEB (worked out) production - 0.0719 Mt @ 3.29% U
3O8 for 2360 t U3O8 recovered.

    McClean North indicated resource - 0.0569 Mt @ 6.16% U
3O8

    Caribou resource at 0.1% U
3O8 cut-off - 0.0478 Mt @ 2.62% U3O8.

The total resource + reserves at Sue A, B, E, McLean North and Caribou (at a 0.1% U
3O8 cut-off), plus past production from JEB and Sue amounts to approximately 32 350 t U3O8.

The most recent source geological information used to prepare this summary was dated: 2006.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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